Hypercatabolism of lipoprotein-free apolipoprotein A-I in HDL-deficient mutant chickens

Cholesterol accumulation in ovarian follicles causes ovulation defects in Abca1a -/- Japanese medaka (Oryzias latipes)

ATP-binding cassette A1 (ABCA1) is a membrane protein, which exports excess cellular cholesterol to generate HDL to reduce the risk of the onset of cardiovascular diseases (CVD). In addition, ABCA1 exerts pleiotropic effects on such as inflammation, tissue repair, and cell proliferation and migration. In this study, we explored the novel physiological roles of ABCA1 using Japanese medaka (Oryzias latipes), a small teleost fish. Three Abca1 genes were found in the medaka genome. ABCA1A and ABCA1C exported cholesterol to generate nascent HDL as human ABCA1 when expressed in HEK293 cells. To investigate their physiological roles, each Abca1-deficient fish was generated using the CRISPR-Cas9 system. Abca1a ^-/- female medaka was found to be infertile, while Abca1b ^-/- and Abca1c ^-/- female medaka were fertile. In vitro ovarian follicle culture suggested that Abca1a deficiency causes ovulation defects. In the ovary, ABCA1A was expressed in theca cells, an outermost layer of the ovarian follicle. Total cholesterol content of Abca1a ^-/- ovary was significantly higher than that of the wild-type, while estrogen and progestin contents were compatible with those of the wild-type. Furthermore, cholesterol loading to the wild-type follicles caused ovulation defects. These results suggest that ABCA1A in theca cells regulates cholesterol content in the ovarian follicles and its deficiency inhibits successful ovulation through cholesterol accumulation in the ovarian follicle.

High-density lipoprotein metabolism in human apolipoprotein B(100) transgenic/brown adipose tissue deficient mice: a model of obesity-induced hyperinsulinemia

Obese and diabetic humans display decreased plasma high-density lipoprotein cholesterol (HDL-C) concentrations and an increased risk for coronary heart disease. However, investigation on HDL metabolism in obesity with a particular emphasis on hepatic ATP-binding cassette transporter A1 (ABCA1), the primary factor for HDL formation, has not been well studied. Human apolipoprotein B(100) transgenic (hApoB(tg)) and brown adipose tissue deficient (BATless) mice were crossed to generate hApoB(tg)/BATless mice. Male and female hApoB(tg) and hApoB(tg)/BATless mice were maintained on either a regular rodent chow diet or a diet high in fat and cholesterol until 24 weeks of age. The hApoB(tg)/BATless mice that were fed a HF/HC diet became obese, developed hepatic steatosis, and had significantly elevated plasma insulin levels compared with their hApoB(tg) counterparts, but plasma concentrations of total cholesterol, HDL-C, triglycerides, and free fatty acids and lipoprotein distribution between genotypes were not significantly different. Hepatic expression of genes encoding HDL-modifying factors (e.g., scavenger receptor, class B, type I, hepatic lipase, lecithin:cholesterol acyltransferase, and phospholipid transfer protein) was either altered significantly or showed a trend of difference between 2 genotypes of mice. Importantly, hepatic protein levels of ABCA1 were significantly lowered by ∼35% in male obese hApoB(tg)/BATless mice with no difference in mRNA levels compared with hApoB(tg) counterparts. Despite reduced hepatic ABCA1 protein levels, plasma HDL-C concentrations were not altered in male obese hApoB(tg)/BATless mice. The result suggests that hepatic ABCA1 may not be a primary contributing factor for perturbations in HDL metabolism in obesity-induced hyperinsulinemia.

Hepatic ATP-binding cassette transporter A1 is a key molecule in high-density lipoprotein cholesteryl ester metabolism in mice

OBJECTIVE

Mutations in ATP-binding cassette transporter A1 (ABCA1), the cellular lipid transport molecule mutated in Tangier disease, result in the rapid turnover of high-density lipoprotein (HDL)-associated apolipoproteins that presumably are cleared by the kidneys. However, the role of ABCA1 in the liver for HDL apolipoprotein and cholesteryl ester (CE) catabolism in vivo is unknown.

METHODS AND RESULTS

Murine HDL was radiolabeled with 125I in its apolipoprotein and with [3H]cholesteryl oleyl ether in its CE moiety. HDL protein and lipid metabolism in plasma and HDL uptake by tissues were investigated in ABCA1-overexpressing bacterial artificial chromosome (BAC)-transgenic (BAC+) mice and in mice harboring deletions of total (ABCA1-/-) and liver-specific ABCA1 (ABCA1(-L/-L)). In BAC+ mice with elevated ABCA1 expression, fractional catabolic rates (FCRs) of both the protein and the lipid tracer were significantly decreased in plasma and in the liver, yielding a diminished hepatic selective CE uptake from HDL. In contrast, ABCA1-/- or ABCA1(-L/-L) mice had significantly increased plasma and liver FCRs for both HDL tracers. An ABCA1 deficiency was associated with increased selective HDL CE uptake by the liver under all experimental conditions.

CONCLUSIONS

Hepatic ABCA1 has a critical role for HDL catabolism in plasma and for HDL selective CE uptake by the liver.

ABCA1 and atherosclerosis

ATP binding cassette transporter A1 (ABCA1) mediates the cellular efflux of phospholipids and cholesterol to lipid-poor apolipoprotein A1 (apoA1) and plays a significant role in high density lipoprotein (HDL) metabolism. ABCA1's role in the causation of Tangier disease, characterized by absent HDL and premature atherosclerosis, has implicated this transporter and its regulators liver-X-receptoralpha (LXRalpha) and peroxisome proliferator activated receptorgamma (PPARgamma) as new candidates potentially influencing the progression of atherosclerosis. In addition to lipid regulation, these genes are involved in apoptosis and inflammation, processes thought to be central to atherosclerotic plaque progression. A Medline-based review of the literature was carried out. Tangier disease and human heterozygotes with ABCA1 mutations provide good evidence that ABCA1 is a major candidate influencing atherosclerosis. Animal and in vitro experiments suggest that ABCA1 not only mediates cholesterol and phospholipid efflux, but is also involved in the regulation of apoptosis and inflammation. The complex and beneficial interactions between apoA1 and ABCA1 seem to be pivotal for cholesterol efflux. The expression of the ABCA1 is tightly regulated. Furthermore the plaque microenvironment could potentially promote ABCA1 protein degradation thus compromising cholesterol efflux. PPAR-LXR-ABCA1 interactions are integral to cholesterol homeostasis and these nuclear receptors have proven anti-inflammatory and anti-matrix metalloproteinase activity. Therapeutic manipulation of the ABCA1 transporter is feasible using PPAR and LXR agonists. PPAR agonists like glitazones and ABCA1 protein stabilization could potentially modify the clinical progression of atherosclerotic lesions.

HDLs in apoA-I transgenic Abca1 knockout mice are remodeled normally in plasma but are hypercatabolized by the kidney

Patients homozygous for Tangier disease have a near absence of plasma HDL as a result of mutations in ABCA1 and hypercatabolize normal HDL particles. To determine the relationship between ABCA1 expression and HDL catabolism, we investigated intravascular remodeling, plasma clearance, and organ-specific uptake of HDL in mice expressing the human apolipoprotein A-I (apoA-I) transgene in the Abca1 knockout background. Small HDL particles (7.5 nm), radiolabeled with (125)I-tyramine cellobiose, were injected into recipient mice to quantify plasma turnover and the organ uptake of tracer. Small HDL tracer was remodeled to 8.2 nm diameter particles within 5 min in human apolipoprotein A-I transgenic (hA-I(Tg)) mice (control) and knockout mice. Decay of tracer from plasma was 1.6-fold more rapid in knockout mice (P < 0.05) and kidney uptake was twice that of controls, with no difference in liver uptake. We also observed 2-fold greater hepatic expression of ABCA1 protein in hA-I(Tg) mice compared with nontransgenic mice, suggesting that overexpression of human apoA-I stabilized hepatic ABCA1 protein in vivo. We conclude that ABCA1 is not required for in vivo remodeling of small HDLs to larger HDL subfractions and that the hypercatabolism of normal HDL particles in knockout mice is attributable to a selective catabolism of HDL apoA-I by the kidney.

Targeted inactivation of hepatic Abca1 causes profound hypoalphalipoproteinemia and kidney hypercatabolism of apoA-I

Patients with Tangier disease exhibit extremely low plasma HDL concentrations resulting from mutations in the ATP-binding cassette, sub-family A, member 1 (ABCA1) protein. ABCA1 controls the rate-limiting step in HDL particle assembly by mediating efflux of cholesterol and phospholipid from cells to lipid-free apoA-I, which forms nascent HDL particles. ABCA1 is widely expressed; however, the specific tissues involved in HDL biogenesis are unknown. To determine the role of the liver in HDL biogenesis, we generated mice with targeted deletion of the second nucleotide-binding domain of Abca1 in liver only (Abca1(-L/-L)). Abca1(-L/-L) mice had total plasma and HDL cholesterol concentrations that were 19% and 17% those of wild-type littermates, respectively. In vivo catabolism of HDL apoA-I from wild-type mice or human lipid-free apoA-I was 2-fold higher in Abca1(-L/-L) mice compared with controls due to a 2-fold increase in the catabolism of apoA-I by the kidney, with no change in liver catabolism. We conclude that in chow-fed mice, the liver is the single most important source of plasma HDL. Furthermore, hepatic, but not extrahepatic, Abca1 is critical in maintaining the circulation of mature HDL particles by direct lipidation of hepatic lipid-poor apoA-I, slowing its catabolism by the kidney and prolonging its plasma residence time.

ATP-binding cassette transporter AI and its role in HDL formation

PURPOSE OF REVIEW

ATP-binding cassette transporter AI (ABCA1)-mediated assembly of phospholipid and free cholesterol with apoA-I plays an important role in HDL biogenesis. This review focuses on recent progress in ABCA1-mediated HDL formation and regulation of ABCA1 expression.

RECENT FINDINGS

Studies of hepatic ABCA1 overexpression suggest that the liver is a major site for HDL formation. Lipidation of apoA-I by ABCA1 increases its potential for reverse cholesterol transport based on the following findings: (1) apoA-I/lipid complexes formed by ABCA1 are better acceptors of cellular lipid via non-ABCA1-mediated efflux pathways than lipid-free apoA-I in vitro and (2) lipidation of apoA-I prevents it from rapidly associating with plasma HDL in vivo, resulting in more available nascent pre-beta HDL for cellular lipid efflux. Several novel regulatory mechanisms for ABCA1 at the post-transcriptional level have been identified recently. Interaction of apoA-I with ABCA1 prevents phosphorylation of a sequence rich in proline, glutamic acid, serine and threonine in a cytoplasmic domain of ABCA1, resulting in less degradation by calpain proteolysis and increased surface expression of ABCA1. In addition, destabilization and decreased cellular surface expression of ABCA1 protein by unsaturated fatty acids have been identified.

SUMMARY

Initial lipidation of apoA-I by hepatic ABCA1 is critical for plasma HDL formation because it enables pre-beta HDL to function more efficiently as a cholesterol acceptor for other pathways of cholesterol efflux in the reverse cholesterol transport pathway and prevents apoA-I from rapidly associating with preexisting plasma HDL particles, resulting in greater availability of pre-beta HDL particles for cholesterol efflux.

The genetics of ATP-binding cassette transporters

The ATP-binding cassette (ABC) superfamily consists of membrane proteins that transport a wide variety of substrates across membranes. Mutations in ABC transporters cause or contribute to a number of different Mendelian disorders, including adrenoleukodystrophy, cystic fibrosis, retinal degeneration, cholesterol, and bile transport defects. In addition, the genes are involved in an increasing number of complex disorders. The proteins play essential roles in the protection of organisms from toxic metabolites and compounds in the diet and are involved in the transport of compounds across the intestine, blood-brain barrier, and the placenta. There are 48 ABC genes in the human genome divided into seven subfamilies based in gene structure, amino acid alignment, and phylogenetic analysis. These seven subfamilies are found in all other sequenced eukaryotic genomes and are of ancient origin. Further characterization of all ABC genes from humans and model organisms will lead to additional insights into normal physiology and human disease.

Prebeta high density lipoprotein has two metabolic fates in human apolipoprotein A-I transgenic mice

We compared the in vivo metabolism of prebeta HDL particles isolated by anti-human apolipoprotein A-I (apoA-I) immunoaffinity chromatography (LpA-I) in human apoA-I transgenic (hA-I Tg) mice with that of lipid-free apoA-I (LFA-I) and small LpA-I. After injection, prebeta LpA-I were removed from plasma more rapidly than were LFA-I and small LpA-I. Prebeta LpA-I and LFA-I were preferentially degraded by kidney compared with liver; small LpA-I were preferentially degraded by the liver. Five minutes after tracer injection, 99% of LFA-I in plasma was found to be associated with medium-sized (8.6 nm) HDL, whereas only 37% of prebeta tracer remodeled to medium-sized HDL. Injection of prebeta LpA-I doses into C57Bl/6 recipients resulted in a slower plasma decay compared with hA-I Tg recipients and a greater proportion (>60%) of the prebeta radiolabel that was associated with medium-sized HDL. Prebeta LpA-I contained one to four molecules of phosphatidylcholine per molecule of apoA-I, whereas LFA-I contained less than one. We conclude that prebeta LpA-I has two metabolic fates in vivo, rapid removal from plasma and catabolism by kidney or remodeling to medium-sized HDL, which we hypothesize is determined by the amount of lipid associated with the prebeta particle and the particle's ability to bind to medium-sized HDL.

The lipidation by hepatocytes of human apolipoprotein A-I occurs by both ABCA1-dependent and -independent pathways

The pathways of hepatic intra- and peri-cellular lipidation of apolipoprotein A-I (apoA-I) were studied by infecting primary mouse hepatocytes from either apoA-I-deficient or ABCA1-deficient mice with a recombinant adenovirus expressing the human apoA-I (hapoA-I) cDNA (endo apoA-I) or incubating the hepatocytes with exogenously added hapoA-I (exo apoA-I) and examining the hapoA-I-containing lipoproteins formed. The cells, maintained in serum-free medium, were labeled with [(3)H]choline, and the cell medium was separated by fast protein liquid chromatography or immunoprecipitated to quantify labeled choline phospholipids specifically associated with hapoA-I. With the apoA-I-deficient hepatocytes, the high density lipoprotein fraction formed with endo apoA-I contained proportionally more phospholipids than that formed with exo apoA-I. However, the lipoprotein size and electrophoretic mobility and phospholipid profiles were similar for exo apoA-I and endo apoA-I. Taken together, these data demonstrate that a significant proportion of hapoA-I is secreted from hepatocytes in a phospholipidated state but that hapoA-I is also phospholipidated peri-cellularly. With primary hepatocytes from ABCA1-deficient mice, the expression and net secretion of adenoviral-generated endogenous apoA-I was unchanged compared with control mice, but (3)H-phospholipids associated with endo apoA-I and exo apoA-I decreased by 63 and 25%, respectively. The lipoprotein size and electrophoretic migration and their phospholipid profiles remained unchanged. In conclusion, we demonstrated that intracellular and peri-cellular lipidation of apoA-I represent distinct and additive pathways that may be regulated independently. Hepatocyte expression of ABCA1 is central to the lipidation of newly synthesized apoA-I but also contributes to the lipidation of exogenous apoA-I. However, a significant basal level of phospholipidation occurs in the absence of ABCA1.

Identification and functional analysis of a naturally occurring E89K mutation in the ABCA1 gene of the WHAM chicken

The Wisconsin hypoalpha mutant (WHAM) chicken has a >90% reduction in plasma HDL due to hypercatabolism by the kidney of lipid-poor apoA-I. The WHAM chickens have a recessive white skin phenotype caused by a single-gene mutation that maps to the chicken Z-chromosome. This corresponds to human 9q31.1, a chromosomal segment that contains the ATP-binding cassette protein-1 (ABCA1) gene, which is mutated in Tangier Disease and familial hypoalphalipoproteinemia. Complete sequencing of the WHAM ABCA1 cDNA identified a missense mutation near the N-terminus of the protein (E89K). The substitution of this evolutionary conserved glutamate residue for lysine in the mouse ABCA1 transporter leads to complete loss of function, resulting principally from defective intracellular trafficking and very little ABCA1 reaching the plasma membrane. The WHAM chicken is a naturally occurring animal model for Tangier Disease.

Very Small Apolipoprotein A-I-containing Particles from Human Plasma: Isolation and Quantification by High-Performance Size-Exclusion Chromatography

Background: Very small apolipoprotein (apo) A-I-containing lipoprotein (Sm LpA-I) particles with pre-β electrophoretic mobility may play key roles as “nascent” and/or “senescent” HDL; however, methods for their isolation are difficult and often semiquantitative.

Methods: We developed a preparative method for separating Sm LpA-I particles from human plasma by high-performance size-exclusion chromatography (HP-SEC), using two gel permeation columns (Superdex 200 and Superdex 75) in series and measuring apo A-I content in column fractions in 30 subjects with HDL-cholesterol (HDL-C) concentrations of 0.4–3.83 mmol/L.

Results: Three major sizes of apo A-I-containing particles were detected: an ∼15-nm diameter (∼700 kDa) species; a 7.5–12 nm (100–450 kDa) species; and a 5.8–6.3 nm species (40–60 kDa, Sm LpA-I particles), containing 0.2–3%, 80–96%, and 2–15% of plasma total apo A-I, respectively. Two subjects with severe HDL deficiency had increased relative apo A-I content in Sm LpA-I: 25% and 37%, respectively. The percentage of apo A-I in Sm LpA-I correlated positively with fasting plasma triglyceride concentrations (r = 0.581; P &lt;0.0005) and inversely with total apo A-I (r = −0.551; P &lt;0.0013) and HDL-C concentrations (r = −0.532; P &lt;0.0017), although the latter two relationships were largely attributable to extremely hypoalphalipoproteinemic subjects. The percentage of apo A-I in Sm LpA-I correlated with that in pre-β-migrating species by crossed immunoelectrophoresis (r = 0.98; P &lt;0.0001; n = 24) and with that in the d &gt;1.21 kg/L fraction by ultracentrifugation (r = 0.86; P &lt;0.001; n = 20). Sm LpA-I particles, on average, appear to contain two apo A-I and four phospholipid molecules but little or no apo A-II, triglyceride, or cholesterol.

Conclusions: We present a new HP-SEC method for size separation of native HDL particles from plasma, including Sm Lp A-I, which may play important roles in the metabolism of HDL and in its contribution(s) to protection against atherosclerosis. This method provides a basis for further studies of the structure and function of Sm Lp A-I.

Gene therapy for lipid disorders

Lipid disorders are associated with atherosclerotic vascular disease, and therapy is associated with a substantial reduction in cardiovascular events. Current approaches to the treatment of lipid disorders are ineffective in a substantial number of patients. New therapies for refractory hypercholesterolemia, severe hypertriglyceridemia, and low levels of high-density lipoprotein cholesterol are needed: somatic gene therapy is one viable approach. The molecular etiology and pathophysiology of most of the candidate diseases are well understood. Animal models exist for the diseases and in many cases preclinical proof-of-principle studies have already been performed. There has been progress in the development of vectors that provide long-term gene expression. New clinical gene therapy trials for lipid disorders are likely to be initiated within the next few years.